POS1081 SERONEGATIVE RHEUMATOID ARTHRITIS (RA) – IS IT REALLY RA?
Authors: E. Birgersson Wettersand, K. Chatzidionysiou
Journal: Annals of the Rheumatic Diseases (2023)

<h3>Background</h3> The 2010 rheumatoid arthritis (RA) classification criteria are intended for research purposes and classification of the disease and not for diagnostic purposes. It is however a tool which clinicians commonly use in everyday practice when evaluating patients with arthritis. It is clearly stated in the criteria that the synovitis should not be better explained by another disease. However, some patients who are classified as seronegative RA might still have an alternative diagnosis, such as psoriatic arthritis, chronic gout or pseudogout, polymyalgia rheumatica etc. Misclassification has direct impact on treatment, prognosis and follow up of patients, but also on research projects. Increased attention and constant re-evaluation should be embraced in this group of patients to avoid misclassification and promote optimal treatment. <h3>Objectives</h3> To assess the frequency of patients classified as seronegative RA that have a potential alternative diagnosis that could better explain the arthritis. <h3>Methods</h3> This is an observational cohort study including adult patients classified as rheumatoid factor (RF) and anti-cyclic citrullinated peptide (anti-CCP) negative RA (symptom duration &lt; 1 year), based on ICD-10-SE (M06.0, M06.0M), and without typical RA radiographical changes, between 2015-2019 at Karolinska University hospital. Patients were identified from the Swedish Quality Rheumatology register (SRQ) and had a minimum of 2 years follow-up after RA diagnosis. Additional medical information was collected from the medical charts retrospectively. Patients were re-classified in 6 different groups: G1: presence of symptoms and signs typical for crystal arthritis (gout or pseudogout), and/or hyperuricemia and/or typical ultrasound and/or x-ray changes; G2: symptoms typical for PMR – fulfilment of PMR criteria at the time of RA diagnosis or before; G3: diagnosis of systemic inflammatory disease after RA diagnosis (SLE, myositis, systemic sclerosis, etc); G4: cancer diagnosis one year before or after RA diagnosis; G5: signs of seronegative spondylarthritis (i.e. diagnosis of psoriasis before or after RA diagnosis, development of inflammatory back pain, presence of dactylitis +/- enthesitis); G6: no other diagnosis than seronegative RA which can better explain the polyarthritis. <h3>Results</h3> A total of 229 patients received a diagnosis of RA and fulfilled the criteria (double-seronegativity and absence of radiographical changes) during 2015-2019 and were included in the study. Approximately 85% of the patients had at least 3 years and 50% at least 5 years of follow-up time. The percentage of patients who were re-classified in the 6 subgroups as described above were: G1 = 6.6%, G2 = 6.5%, G3 =3.1%, G4 =7.4%, G5 = 4.4% and G6 =72%. <h3>Conclusion</h3> In this observational study almost one third of patients who received a diagnosis of seronegative RA have an alternative diagnosis than RA that can explain the arthritis, with paramalignant phenomenon, PMR and crystal arthritis being the most common. Increased vigilance in the subgroup of seronegative, non-erosive RA patients is important and constant consideration of differential diagnoses is warrantied. <h3>References</h3> [1] Aletaha D, Neogi T, Silman AJ, et al. 2010 Rheumatoid arthritis classification criteria: an American College of Rheumatology/European League Against Rheumatism collaborative initiative. Arthritis Rheum. 2010 Sep;62(9):2569-81. [2] Paalanen K, Puolakka K, Nikiphorou E, et al. Is seronegative rheumatoid arthritis true rheumatoid arthritis? A nationwide cohort study. Rheumatology (Oxford). 2021 May 14;60(5):2391-2395. doi: 10.1093/rheumatology/keaa623. Erratum in: Rheumatology (Oxford). 2021 May 14;60(5):2493-2494. PMID: 33175968; PMCID: PMC8121445. <h3>Acknowledgements:</h3> NIL. <h3>Disclosure of Interests</h3> Emma Birgersson Wettersand: None declared, Katerina Chatzidionysiou Grant/research support from: Research grant: Galapagos.